The present invention relates to a process for producing a 3-hydroxytetrahydrofuran, particularly an optically active 3-(S)-hydroxytetrahydrofuran of high quality, in a simple, easy and industrially advantageous manner and in high yield.
3-Hydroxytetrahydrofuran is a compound of great value as a synthetic intermediate of medicinals and agrochemicals. The known technology for the production thereof comprises reducing a 4-halo-3-hydroxybutyric acid ester, which is readily available, and cyclizing the resulting 4-halo-1,3-butanediol. Specifically, production processes of 3-hydroxytetrahydrofuran are known which comprise reducing a 4-halo-3-hydroxybutyric acid ester with sodium borohydride in an organic solvent miscible with water, such as tetrahydrofuran (hereinafter referred to as THF), and cyclizing the resulting 4-halo-1,3-butanediol in aqueous hydrochloric acid to give 3-hydroxytetrahydrofuran [e.g. Japanese Kokai Publication Hei-09-77759; Liebigs Ann./Recl., page 1877 (1997)].
The steps involved in a typical production process among these as described are as follows:
Step 1: Reducing ethyl 4-chloro-3-hydroxybutyrate with sodium borohydride in THF;
Step 2: Adding aqueous hydrochloric acid to the concentration residue of the reaction mixture, extracting the mixture with ethyl acetate, dehydrating the extract with a solid desiccant, separating the desiccant by filtration and removing the extractant solvent by concentration, to give 4-chloro-1,3-butanediol as an oil;
Step 3: Dissolving this oil in aqueous hydrochloric acid and heating the solution to cause cyclization;
Step 4: Neutralizing the reaction mixture, removing water by concentration, adding methanol to the residue and removing the precipitated inorganic salt by filtration; and
Step 5: Concentrating and distilling the filtrate to recover 3-hydroxytetrahydrofuran.
However, such a process requires complicated procedural steps such as a plurality of concentration and solid-liquid separation procedures. If a concentration procedure is omitted and water is directly added to the reaction mixture, a mixed solvent composed of the solvent and water will be formed, since the solvent is miscible with water. Investigations made by the present inventors revealed that if the cyclization reaction is then carried out in such a solvent, unfavorable results are inevitable, for example the rate of reaction decreases and the impurity content increases. Therefore, in carrying out the reduction reaction using a organic solvent miscible with water, it is essential to interpose a step of removing this organic solvent prior to addition of water to the reaction mixture. The prior art processes are thus low in productivity and cannot be said to be fully suited for commercial production.
Furthermore, in the prior art processes, generally the yield in the reduction step is 86% and the yield in the cyclization step is 68 to 79%, hence the yield of the desired product is 58 to 68%. These figures are not necessarily satisfactory. The prior art processes have further problems with respect to the quality of the desired product; it is known that, in the reduction reaction, 3,4-epoxy-1-butanol is formed as a byproduct (the byproduct yield being about 16 to 21%; Japanese Kokai Publication Hei-02-174733) and, in the cyclization reaction, 2,5-dihydrofuran is formed as a byproduct [the byproduct yield being about 15%; Liebigs Ann./Recl., page 1877 (1997)].
As mentioned above, in the state of art, no simple, easy and efficient processes are known, to say nothing of processes that may be carried out on a commercial scale with advantage, for producing high-quality 3-hydroxytetrahydrofuran in high yield by reduction of a 4-halo-3-hydroxybutyric acid ester and cyclization of the resulting 4-halo-1,3-butanediol.
In view of the above state of the art, it is an object of the present invention to provide a simple, easy and industrially advantageous process for producing high-quality 3-hydroxytetrahydrofuran (3) by reducing a 4-halo-3-hydroxybutyric acid ester (1) and cyclizing the resulting 4-halo-1,3-butanediol (2).
The present invention provides a process for producing a 3-hydroxytetrahydrofuran of the general formula (3): 
by reducing a 4-halo-3-hydroxybutyric acid ester of the general formula (1): 
xe2x80x83wherein R represents an ester-forming protective group and X represents a halogen atom
and cyclizing the resulting 4-halo-1,3-butanediol of the general formula (2): 
xe2x80x83wherein X represents a halogen atom which comprises:
Step 1: Reducing a 4-halo-3-hydroxybutyric acid ester (1) with a boron hydride compound and/or an aluminum hydride compound as a reducing agent in an organic solvent immiscible with water;
Step 2: Treating the obtained reaction mixture with an acid and water to thereby effect conversion to the corresponding 4-halo-1,3-butanediol (2) and at the same time giving an aqueous solution containing said compound;
Step 3: Carrying out the cyclization reaction of the 4-halo-1,3-butanediol (2) in said aqueous solution;
Step 4: Extracting the resulting 3-hydroxytetrahydrofuran (3) from the obtained aqueous solution containing 3-hydroxytetrahydrofuran with an organic solvent immiscible with water; and
Step 5: Isolating the 3-hydroxytetrahydrofuran (3) by concentration and/or distillation of the solution obtained.
The present invention also provides a process for producing a 4-halo-1,3-butanediol
which comprises reducing a 4-halo-3-hydroxybutyric acid ester (1) with a boron hydride compound and/or an aluminum hydride compound as a reducing agent in an organic solvent immiscible with water
and treating the obtained reaction mixture with an acid and water to thereby effect conversion to the corresponding 4-halo-1, 3-butanediol (2) and at the same time giving an aqueous solution containing said compound.
The present invention further provides a process for producing a 3-hydroxytetrahydrofuran (3) by cyclizing a 4-halo-1,3-butanediol (2) in an aqueous solution
which comprises carrying out the cyclization reaction under weakly acidic to neutral conditions.
The present invention further provides a process for recovering a 3-hydroxytetrahydrofuran
which comprises extracting 3-hydroxytetrahdyrofuran from an aqueous solution containing 3-hydroxytetrahydrofuran with an organic solvent immiscible with water at a temperature not lower than 40xc2x0 C.
The present invention further provides a process for recovering a 3-hydroxytetrahydrofuran
which comprises extracting 3-hydroxytetrahydrofuran from an aqueous solution containing 3-hydroxytetrahydrofuran with a monohydric alcohol containing 4 to 8 carbon atoms.
The present invention further provides a process for recovering a 3-hydroxytetrahydrofuran from a mixture comprising 3-hydroxytetrahydrofuran and a boron compound and/or an aluminum compound by distillation
which comprises treating the mixture comprising 3-hydroxytetrahydrofuran and a boron compound and/or an aluminum compound with a monohydric alcohol containing 1 to 3 carbon atoms or a polyhydric alcohol containing at least 6 carbon atoms in carrying out the distillation.
Lastly, the present invention provides a process for recovering a 3-hydroxytetrahydrofuran
which comprises carrying out distillation (inclusive of rectification) of 3-hydroxytetrahydrofuran in the presence of a base.
In the following, the present invention is described in detail.
The five-step process for producing a 3-hydroxytetrahydrofuran is described below in detail. The processes according to other aspects of the present invention can also be carried out by the same techniques mentioned hereinbelow.
In the first step of this production process, a 4-halo-3-hydroxybutyric acid ester represented by the above general formula (1) [hereinafter referred to also as xe2x80x9c4-halo-3-hydroxybutyric acid ester (1)xe2x80x9d] is reduced with a boron hydride compound and/or an aluminum hydride compound in an organic solvent immiscible with water to cause formation of the corresponding 4-halo-1,3-butanediol represented by the above general formula (2) [hereinafter referred to also as xe2x80x9c4-halo-1,3-butanediol (2)xe2x80x9d].
The symbol R in the above general formula (1) represents an ester-forming protective group. The term xe2x80x9cester-forming protective groupxe2x80x9d as used herein means a group capable of protecting a carboxylic acid by forming an ester therewith. The ester-forming protective group is not particularly restricted. Thus, it may be a conventional ester-forming protective group, preferably an alkyl group, more preferably an alkyl group containing 1 to 4 carbon atoms, most preferably an ethyl group.
The symbol X in the above general formulas (1) and (2) represents a halogen atom, which is a leaving atom capable of leaving under formation of an ether bond between the hydroxy group at position 1 and the carbon atom at position 4 in the general formula (2). It is preferably chlorine, bromine or iodine, more preferably chlorine.
The reaction substrate, namely 4-halo-3-hydroxybutyric acid ester (1) is generally prepared by reducing a 4-haloacetoacetic acid ester, which is readily available. For obtaining an optically active form of (1), which is useful as a raw material for the synthesis in the pharmaceutical and agrochemical field, in particular, methods are known which use a chemical agent capable of asymmetric reduction, or a microorganism or an enzyme. Thus, said form can be prepared by the methods described in Japanese Kokai Publication Hei-01-211551, J. Am. Chem. Soc., vol. 105, p. 5925 (1983) and Japanese Kokoku Publication Hei-04-7195, for instance. In the production process according to the present invention, this compound, when it is optically active, can give the 4-halo-1,3-butanediol (2) and 3-hydroxytetrahydrofuran (3) while the optical activity is retained. When, for example, the process of the present invention is carried out using a 4-halo-3-(S)-hydroxybutyric acid ester, 3-(S)-hydroxytetrahydrofuran can be obtained with high purity and in high yield.
The reducing agent to be used is a boron hydride compound and/or an aluminum hydride compound. More specifically, the reducing agent includes but is not limited to alkali metal borohydrides, alkaline earth metal borohydrides, alkali metal aluminum hydrides, dialkylaluminum hydrides and diborane, among others. These may be used singly or two or more of them may be used in a suitable combination. The salt-forming alkali metal in the reducing agent is, for example, lithium, sodium or potassium and the alkaline earth metal is calcium or magnesium. In consideration of the ease of handling and from other viewpoints, alkali metal borohydrides are preferred, and sodium borohydride is particularly preferred. Together with such a reducing agent, an activator generally known in the art may be combinedly used for improving the reducing power of the reducing agent.
The amount of the reducing agent is not particularly restricted but is preferably such that hydrogen is provided in an amount not less than the stoichiometric amount relative to the 4-halo-3-hydroxybutyric acid ester (1). For example, the reduction can be effected using sodium borohydride in an amount of not less than 0.5 mole, preferably not less than 0.75 mole, per mole of the 4-halo-3-hydroxybutyric acid ester (1). From the economic viewpoint, said amount is preferably not more than 10.0 moles, more preferably not more than 5.0 moles, still more preferably not more than 2.0 moles.
The concentration of the 4-halo-3-hydroxybutyric acid ester (1) in the reaction mixture cannot be specifically defined since it may vary according to the kind of the reaction solvent employed. Generally, however, it may, for example, be 1 to 50% by weight, preferably 5 to 40% by weight, more preferably 10 to 30% by weight.
The reaction temperature cannot be specifically defined since it depends on the reducing agent and reaction solvent employed. Generally, however, it is within the range between the solidifying point and the boiling point of the reaction solvent employed, preferably 20 to 80xc2x0 C. For causing the reaction to proceed efficiently and, in particular, for driving the reaction to completion in a period of time suited for a commercial scale production, a temperature of not lower than 40xc2x0 C. is preferred. On the other hand, for suppressing side reactions or decomposition, it is important that the reaction temperature is not excessively high. For example, it is preferred that the reaction is carried out at a temperature not higher than 80xc2x0 C., preferably not higher than 60xc2x0 C. The reaction time is generally about 24 hours at longest.
The reduction reaction according to the invention is an exothermic reaction and is accompanied with rapid heat generation particularly in the initial stage thereof. It is, therefore, important to adequately control the reaction so that it may proceed smoothly. From this point of view, the reaction is preferably carried out by adding the 4-halo-3-hydroxybutyric acid ester (1) and/or the above reducing agent in a continuous manner or intermittently in portions. Specifically, the reaction may be carried out while adding either of the 4-halo-3-hdyroxybutyric acid ester (1) or the reducing agent or while adding both compounds simultaneously. For carrying out the reduction reaction in a simple and safe manner on a commercial scale, it is generally preferred that the reducing agent is gradually added to the solution of the 4-halo-3-hydroxybutyric acid ester (1). The duration of this addition is preferably not less than 1 hour, more preferably not less than 2 hours, still more preferably not less than 5 hours.
The organic solvent to be used as the solvent in this reduction reaction, which is immiscible with water, is generally an organic solvent having a physical property such that when it is gently stirred together with an equal volume of pure water under a pressure of one atmosphere and at a temperature of 20xc2x0 C., the resulting mixture, after stopping of the flow, shows a heterogeneous appearance. The solubility in water is not particularly restricted but an organic solvent having a solubility in water of not more than 30% by weight, in particular not more than 10% by weight, is generally preferred, and an organic solvent having a solubility in water of not more than 5% by weight, in particular not more than 1% by weight, is more preferred.
The above organic solvent immiscible with water preferably has a boiling point not lower than 40xc2x0 C., more preferably not lower than 50xc2x0 C. When a solvent having a boiling point lower than 40xc2x0 C. is used, the reduction reaction temperature cannot be raised, so that the reaction time may be prolonged or the rapid heat generation due to the reduction reaction may readily result in bumping, among other troubles, making it difficult to duly control the reaction. These and other problems handicap the process in productivity and safety, which are especially important in commercial-scale production.
As concrete examples of the organic solvent immiscible with water which are preferred from the above point of view, there may be mentioned, among others, aromatic hydrocarbons such as benzene, toluene, xylene and ethylbenzene; acetic acid esters such as methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate and tert-butyl acetate; aliphatic hydrocarbons such as hexane, cyclohexane and methylcyclohexane; halogenated hydrocarbons such as methylene chloride and chloroform; and ethers such as diisopropyl ether, methyl tert-butyl ether and ethylene glycol dibutyl ether. Among them, hydrocarbons (in particular aromatic hydrocarbons) and acetic acid esters are preferred, and toluene and acetic acid C1-C4 alkyl esters are more preferred. These organic solvents may be used singly or two or more of them may be used combinedly. Aprotic organic solvents immiscible with water are also preferred as the organic solvent immiscible with water.
When the reduction reaction is carried out using the above solvent, the formation of those impurities which are known in the art, such as 3,4-epoxy-1-butanol (Japanese Kokai Publication Hei-02-174733), can be prevented and the 4-halo-1,3-butanediol (2) can be produced in high yields.
In the second step of the production process according to the present invention, the reaction mixture obtained in step 1 is treated using an acid and water to thereby cause formation of the 4-halo-1,3-butanediol (2) and causing the same to transfer to the aqueous phase; an aqueous solution containing the same is thus obtained.
The acid to be used in this step is not particularly restricted but, from the practicality viewpoint, inorganic acids, in particular hydrochloric acid and sulfuric acid, are preferred. These may be used singly or two or more of them may be used combinedly. An aqueous solution obtained by diluting concentrated hydrochloric acid or concentrated sulfuric acid with water may be used as the acid and water.
The acid is used in an amount sufficient to neutralize the basic component resulting from the reducing agent used in step 1 and convert the same to an inorganic salt, that is to say generally in an at least equimolar amount relative to the reducing agent. The reaction mixture is preferably rendered neutral to acidic by using the acid in an amount at least equimolar to the reducing agent.
The amount of water is preferably such that the 4-halo-1,3-butanediol (2) produced can transfer to the aqueous phase to a satisfactory extent.
The treatment of the reaction mixture obtained in step 1 with an acid and water is carried out generally by admixing the reaction mixture simultaneously with the acid and water. It is also possible to first mix the reaction mixture with the acid and then mix the resulting mixture with water. The method of such mixing is not particularly restricted. Thus, the acid can be added to the reaction mixture or preferably the reaction mixture is added to the acid. Such procedure is preferably carried out at a low temperature, preferably at a temperature between room temperature and the freezing point of the solvent.
Since, in the process according to the invention, an organic solvent immiscible with water is used in step 1, the addition of the acid and water to the reaction mixture results in separation into two phases, of which the aqueous phase as it is can be used in step 3. By removing the organic phase by liquid-liquid separation, the aqueous solution containing the 4-halo-1,3-butanediol (2) can be obtained with ease. Further, it is also possible to extract the organic layer separated with water to thereby further increase the yield.
For increasing the yield, the separation of the organic phase by liquid-liquid separation is preferably carried out at a low temperature. By doing so, the transfer of the 4-halo-1,3-butanediol (2) to the organic phase can be prevented and the yield can be prevented from decreasing. More specifically, the operation temperature is preferably within the range between 30xc2x0 C. and the freezing point of the solvent, more preferably at a temperature not higher than 20xc2x0 C., still more preferably not higher than 10xc2x0 C.
To separate and remove the organic phase in that manner is preferred also from the viewpoint of further reducing the amount of trace impurities and byproducts occurring in the reaction mixture obtained in step 1. It is also possible, however, to collect the aqueous solution by concentrating the organic layer to thereby remove the organic solvent. In the aqueous solution obtained in this step, there may coexist the organic phase in an amount of the range not adversely affecting the cyclization reaction in step 3 and, in that case, the organic phase and aqueous phase may form a binary system.
Then, in step 3, the cyclization reaction of the 4-halo-1,3-butanediol (2) is effected in the aqueous solution obtained instep 2. Although the boron compound and/or aluminum compound resulting from decomposition of the reducing agent used in step 1 and another inorganic salt or the like coexist in the aqueous solution obtained in step 2, this cyclization reaction can give 3-hydroxytetrahydrofuran (3) in a high yield.
Even when the cyclization reaction is carried out in the two-phase system in which the organic solvent immiscible with water coexists, namely without removing that organic solvent immiscible with water in step 2, the cyclization reaction can proceed smoothly and give 3-hydroxytetrahydrofuran (3) in a high yield. This reaction mode is also effective in transferring impurities to the organic phase to thereby reduce the impurity concentration in the aqueous phase in which the cyclization reaction proceeds substantially predominantly, hence to thereby prevent side reactions and reduce the amount of byproduct impurities. Furthermore, that reaction mode is advantageous in that since the distribution coefficient of the cyclization product 3-hydroxytetrahydrofuran (3) in the organic solvent immiscible with water is generally lower as compared with the precyclization compound 4-halo-1,3-butanediol (2), a higher yield can be obtained at a small loss of the desired product into the organic phase even when the removal of the organic phase is carried out after the cyclization reaction as compared with before the cyclization reaction.
The operation temperature during the cyclization reaction is not particularly restricted. While the cyclization reaction can proceed even at around room temperature, it is preferred that the reaction is carried out with heating at a temperature not lower than room temperature so that the rate of reaction may be increased and the reaction may be driven to completion in a shorter time. Thus, it is preferably carried out at not lower than 40xc2x0 C., more preferably at not lower than 50xc2x0 C., still more preferably at not lower than 70xc2x0 C. Generally, it is advantageous to carry out the reaction by raising the temperature to the vicinity of the boiling point of the reaction system.
This cyclization reaction is preferably carried out under acidic to neutral conditions. Although the cyclization reaction can proceed under basic conditions as well, basic conditions are generally unfavorable because of a tendency toward formation of impurities. Generally, therefore, the reaction is preferably started under neutral to acidic conditions. However, the reaction mixture is gradually acidified by the acid component (e.g. hydrogen halide) formed with the progress of the cyclization reaction. Since an excessively strong acidity may readily cause impurity formation, it is preferred that the cyclizareaction reaction is started under neutral conditions, for instance, so that the adverse effects of the acid component may be minimized. In this way, the formation of 2,5-dihydrofuran as a byproduct, which is readily formed when the cyclization reaction is started under strongly acidic conditions, as known in the art [Liebigs Ann./Recl., p. 1877 (1997)], can be suppressed and a higher yield of 3-hydroxytetrahydrofuran can be obtained.
Still more preferably, from the viewpoint of minimizing the adverse effects of the acid in the cyclization reaction, the cyclization reaction is carried out under weakly acidic to neutral conditions while maintaining an appropriate acidity by neutralizing, with a base, the acid component (e.g. hydrogen halide) formed with the progress of the reaction, whereby 3-hydroxytetrahydrofuran (3) can be produced with the highest yield and quality. The appropriate acidity in that case varies according to the operation temperature and concentration in the cyclization reaction, the coexisting inorganic salt and other species and amounts thereof, hence can never be specifically defined. Preferably, however, the reaction is carried out while neutralizing the acid component to a pH range of 2 to 7, more preferably 2 to 6.
The base to be used for neutralizing the above acid component is not particularly restricted but includes, among others, inorganic bases such as alkali metal or alkaline earth metal hydroxides, carbonates and hydrogen carbonates; and organic bases such as secondary amines, tertiary amines and quaternary ammonium hydroxides. More specifically, it includes but is not limited to alkali metal hydroxides such as sodium hydroxide, potassium hydroxide and lithium hydroxide; alkali metal carbonates such as sodium carbonate, potassium carbonate and lithium carbonate; alkali metal hydrogen carbonates such as sodium hydrogen carbonate and potassium hydrogen carbonate; alkaline earth metal hydroxides such as magnesium hydroxide and calcium hydroxide; alkaline earth metal carbonates such as calcium carbonate and barium carbonate; secondary amines such as dimethylamine, diethylamine, diisopropylamine and dicyclohexylamine; tertiary amines such as triethylamine, tripropylamine, tributylamine, triamylamine, pyridine and N-methylmorpholine; and quaternary ammonium hydroxides such as tetramethyl-, tetraethyl-, tetrapropyl-, tetrabutyl-, tetraamyl-, tetrahexyl- and benzyltrimethylammonium hydroxides. Preferred, among these bases, from the viewpoints of inexpensiveness, ease of handling and ease of waste water treatment, among others, are inorganic bases, in particular alkali metal hydroxides such as sodium hydroxide, potassium hydroxide and lithium hydroxide, in particular sodium hydroxide and potassium hydroxide.
From the operability viewpoint, the above inorganic base is preferably used in the form of an aqueous solution. For example, an aqueous solution of an alkali metal hydroxide having a concentration of 2 to 20 N, preferably 5 to 20 N, is used with advantage. The base species mentioned above may be used singly or two or more of them may be used combinedly. The reaction may be carried out while adding these bases gradually at a rate such that the reaction mixture may be maintained at an appropriate pH. It is also possible to add sodium carbonate, barium carbonate, calcium carbonate or disodium hydrogen phosphate to thereby utilize the pH buffer action thereof.
The concentration of the 4-halo-1,3-butanediol (2) in the aqueous solution in carrying out the cyclization reaction is not restricted. An excessively high concentration, however, is not preferred since the rate of reaction decreases. Generally, it is 1 to 50% by weight, preferablyl to 35% by weight, more preferably 1 to 20% by weight. By carrying out the cyclization reaction while maintaining the acidity at a proper level, as mentioned above, it is possible to increase the reaction concentration and carry out the cyclization reaction favorably at a concentration of 1 to 30% by weight, preferably 5 to 30% by weight.
In cases that 3-hydroxytetrahydrofuran (3) is used in such fields where it is desirable the slight impurity content, which is already slight, is reduced still further for the production of products of higher quality, for example medicinals, it is also preferred that the aqueous solution obtained in step 3 is washed with an organic solvent immiscible with water. This is effective in further reducing the slight impurity content in the aqueous phase and thus improving the quality of the product 3-hydroxytetrahydrofuran (3).
When the cyclization reaction is carried out in a two-phase system in which water and an organic solvent immiscible with water coexist, mere recovery of the aqueous solution containing 3-hydroxytetrahydrofuran (3) after the cyclization reaction by removal of the organic phase by liquid-liquid separation can result in reduction of impurities from the aqueous phase, hence in quality improvement.
These procedures (washing and organic phase removal by liquid-liquid separation) are preferably carried out at low temperatures. By this, the distribution ratio of 3-hydroxytetrahydrofuran (3) in the organic layer, hence the loss thereof in the organic phase, can be reduced and the yield can be increased accordingly. Specifically, the procedures are preferably carried out at an operation temperature lowered to 30xc2x0 C. or below, more preferably to 20xc2x0 C. or below, still more preferably to a level of 10xc2x0 C. or below down to the freezing point of the solvent.
In step 4, 3-hydroxytetrahydrofuran (3) is extracted from the aqueous solution containing 3-hydroxytetrahydrofuran (3) obtained in step 3, using an organic solvent immiscible with water. By this extraction procedure, the inorganic salt(s) and reducing agent-derived boron compound or aluminum compound, which coexist in the aqueous solution, can be separated from the desired product to give an organic solution containing 3-hydroxytetrahydrofuran (3).
The organic solvent immiscible with water to be used here is not particularly restricted. For example, it maybe selected from among those organic solvents immiscible with water which can be used in the reduction reaction mentioned above. In addition to them, monohydric alcohols containing 4 to 8 carbon atoms, for example butanols such as 1-butanol, 2-butanol and isobutanol, can be used. Preferred are aromatic hydrocarbons, acetic acid esters and monohydric alcohols containing 4 to 8 carbon atoms. More preferred are acetic acid esters, in particular acetic acid C1-C4 alkyl esters, most preferably ethyl acetate. These organic solvents may be used singly or two or more of them may be used combinedly.
Generally, the aqueous solution obtained in step 3 is in an acidified state. It may be subjected to extraction either as it is under acid conditions or after neutralization with a base. It is generally preferred that the extraction procedure is carried out under neutral conditions. The base to be used for the neutralization is not particularly restricted but includes, among others, alkali metal hydroxides such as sodium hydroxide and potassium hydroxide; alkaline earth metal hydroxides such as calcium hydroxide and magnesium hydroxide; alkali metal hydrogen carbonates such as sodium hydrogen carbonate and potassium hydrogen carbonate; alkali metal carbonates such as sodium carbonate and potassium carbonate; alkaline earth metal carbonates such as magnesium carbonate and calcium carbonate; and so forth. Aqueous ammonia and organic bases such as triethylamine, pyridine and other amines may also be used. These bases may be used singly or two or more of them may be used combinedly.
It is also preferred that the aqueous solution obtained in step 3 is once basified with a base, then neutralized with an acid and subjected to extraction. By such treatments, the impurities contained in the solution are converted to impurities readily transferring to the aqueous phase and, which remain in the aqueous phase on the occasion of extraction following neutralization, the purity of the desired product extracted can be increased. This basification can be carried out using one of the bases mentioned above. For attaining increased treatment effects, the pH is preferably adjusted to not less than 10, more preferably not less than 12, though such level is not absolutely necessary. These procedures can be performed at a temperature not lower than the freezing point of the solvent and the time required therefor can be shortened by raising the temperature within the range from room temperature to the boiling point of the solvent. The treatment time depends on the pH and temperature but generally is about 0.1 to 24 hours. After treatment with a base, the aqueous phase is preferably subjected to extraction after neutralization.
The extraction operation temperature in step 4 cannot be specifically defined since it varies depending on the organic solvent employed but the extraction can be effected within the range of the solidifying point to the boiling point of the solvent employed, generally at 20 to 100xc2x0 C. High temperature extraction is particularly preferred, however. Thus, the extraction procedure is performed preferably at 40xc2x0 C. or above, more preferably at 50xc2x0 C. or above, most preferably at 60xc2x0 C. or above. Particularly when a hydrocarbon or acetic acid ester or the like is used as the solvent, high temperature extraction is preferred for increasing the extraction efficiency.
By using such a monohydric alcohol containing 4 to 8 carbon atoms as mentioned above as the solvent, a high extraction effect can be produced even at around room temperature. It is also possible, however, to carry out the extraction at an elevated liquid temperature to further increase the extracting effect. Specifically, it is also possible to carry out the extraction using a monohydric alcohol containing 4 to 8 carbon atoms at a temperature of 40xc2x0 C. or above.
In step 5, the extract solution obtained in step 4 is concentrated and/or distilled to isolate 3-hydroxytetrahydrofuran (3). Thus, the extract is concentrated to thereby remove the organic solvent or the extract is subjected to distillation (inclusive of rectification), to thereby isolate 3-hydroxytetrahydrofuran (3). It is also possible to concentrate the extract to thereby remove the solvent and then further distill (or rectify) the concentrate to thereby isolate 3-hydroxytetrahydrofuran (3). In this case, the residue after concentration as it is can be heated under reduced pressure to effect distillation.
In purifying and isolating 3-hydroxytetrahydrofuran (3) by distillation (inclusive of rectification), the reducing agent-derived boron and/or aluminum compounds occurring in slight amounts in the extract obtained in step 4 may lower the distillation yield. Therefore, for maximizing the final distillation yield, it is preferred that, in carrying out the distillation in step 5, the solution and/or concentrate containing 3-hydroxytetrahydrofuran (3) is treated with an alcohol to thereby remove those impurities in advance. By adding a monohydric alcohol containing 1 to 3 carbon atoms, such as methanol and ethanol, for instance, it is possible to form compounds lower in boiling point than 3-hydroxytetrahydrofuran (3) and distill off these previously. On the other hand, by adding a polyhydric alcohol containing not less than 6 carbon atoms, such as polyethylene glycol and sorbitol, it is possible to cause compounds higher in boiling point than 3-hydroxytetrahydrofuran (3) to remain. This step can be used with advantage for purifying and isolating 3-hydroxytetrahydrofuran by distillation when this has been contaminated with the boron compound and/or aluminum compound mentioned above.
The amount of use of such alcohol should vary with the kind of alcohol and the kinds and amounts of contaminants derived from the reducing agent and cannot be defined in general terms. Generally, however, the alcohol is used in at least an equimolar amount relative to the amount of said contaminants derived from the reducing agent. For an improved treatment effect, it is rather preferred to use the alcohol in excess taking the influence of the water concomitantly present into consideration, particularly when a monohydric alcohol containing 1 to 3 carbon atoms is used. Specifically, a monohydric alcohol containing 1 to 3 carbon atoms, for instance, is used in an amount of not less than 50% by weight, preferably not less than the same weight %, based on the weight of 3-hydroxytetrahydrofuran (3). A polyhydric alcohol containing not less than 6 carbon atoms is used for the above treatment in an amount of not less than 20% by weight, more preferably not less than 30% by weight. In the above manner, the coexisting boron compounds and/or aluminum compounds can be effectively removed. It is preferred that the residual amount of boron compounds and/or aluminum compounds after the treatment be not more than 10 mole percent, more preferably not more than 5 mole percent, per mole of 3-hydroxytetrahydrofuran. Then, the distillation can favorably be conducted and the distillation yield can be maximized.
In cases that 3-hydroxytetrahydrofuran (3) is purified by distillation (inclusive of rectification) on an industrial scale, a prolonged heating operation is required and, therefore, the related impurities contained in slight amounts, such as the 4-halo-3-hydroxybutyric acid ester (1) and/or 4-halo-1,3-butanediol (2) and their related compounds, may gradually undergo thermal decomposition and the resulting decomposition products may find their way into the desired product fraction during distillation, tending to cause problems such as quality deterioration. It was found that, upon these thermal decompositions, acid components are released and acidification progresses accordingly, leading to a tendency toward promoted thermal decomposition. From this point of view, it is also preferred to use an apparatus in which the heat hysteresis can be reduced and the distillation can be carried out while suppressing the thermal decomposition, for example a wiped film evaporator.
However, when the distillation is carried out using an ordinary distillation column in common use, it is good practice to treat the solution containing 3-hydroxytetrahydrofuran (3) and/or a concentrate thereof with an acid, for instance, preferably to add an acid followed by heating before distillation to promote thermal decomposition. This practice inhibits the formation of impurities in the course of distillation to thereby contribute to an improved purification effect of the distillation in a stable manner. It is preferred that a solvent is used in this acid treatment. Thus, it is also preferred to carry out the acid treatment in solution in a monohydric alcohol containing 1 to 3 carbon atoms prior to the distillation step.
The acid to be added is not particularly restricted but may be an inorganic acid or an organic acid. From the practicality viewpoint, inorganic acids, in particular hydrochloric acid and sulfuric acid, are preferred. These may be used singly or two or more of them may be used combinedly. The addition amount of the acid is not particularly restricted but is preferably not less than 0.1% by weight, more preferably not less than 0.2% by weight, still more preferably not less than 0.5% by weight, relative to 3-hydroxytetrahydrofuran (3). The operation temperature in heating treatment following addition of an acid is not particularly restricted but is, for example, room temperature up to the boiling point of the system employed. The time of acid treatment is generally not less than 0.1 hour, preferably not less than 0.5 hour.
The distillation (inclusive of rectification) of 3-hydroxytetrahydrofuran (3) for purification is also preferably carried out in the presence of a base. This is effective in neutralizing the acid components released upon thermal decomposition of such coexisting impurities as mentioned above and increasing the distillation/purification effect while suppressing the influences of the acid components. Furthermore, it contributes significantly to an increased heat stability of 3-hydroxytetrahydrofuran (3). By conducting the distillation mentioned above (inclusive of rectification) for purification, the quality and yield of the desired product can be increased.
The base to be used in that case is not particularly restricted but preferably is a base higher in boiling point than 3-hydroxytetrahydrofuran (3), more preferably an inorganic base. Specifically, there may be mentioned alkali metal hydroxides such as sodium hydroxide and potassium hydroxide; alkaline earth metal hydroxides such as calcium hydroxide and magnesium hydroxide; alkali metal hydrogen carbonates such as sodium hydrogen carbonate and potassium hydrogen carbonate; alkali metal carbonates such as sodium carbonate and potassium carbonate; alkaline earth metal carbonates such as magnesium carbonate and calcium carbonate; and so on. Alkali metal hydrogen carbonates and alkali metal carbonates are preferred, alkali metal hydrogen carbonates, in particular sodium hydrogen carbonate, are preferred. These may be used singly or two or more maybe used combinedly. The addition amount of these bases is not particularly restricted. Generally, the bases are used in an amount of 0.1 to 30% by weight based on the material to be distilled but, from the viewpoint of economy and/or operability, the use thereof in an amount of not more than 10% by weight is preferred, more preferably not more than 5% by weight, still more preferably not more than 2% by weight.
A preferred embodiment of the present invention in which ethyl 4-chloro-3-(S)-hydroxybutyrate is used as the reaction substrate may be follows.
Ethyl 4-chloro-3-(S)-hydroxybutyrate is reduced with sodium borohydride in toluene, and the reaction mixture is mixed with aqueous hydrochloric acid to give an aqueous solution of 4-chloro-1,3-(S)-butanediol. This aqueous solution is heated as it is to thereby effect the cyclization reaction, preferably while adequately neutralizing the acid component resulting from the reaction, followed by extraction with ethyl acetate under warming, further followed by concentration and/or distillation, to give 3-(S)-hydroxytetrahydrofuran.